PROJECT SUMMARY Parkinson disease (PD) is one of the most disabling chronic health conditions affecting older adults globally. While advances in medical and surgical management of PD have increased lifespans, they have not effectively altered the progressive decline in physical function and quality of life associated with PD. Identifying effective ways to improve function, slow decline and prevent or reduce disability remains of utmost importance in PD. Of particular concern in PD is gait decline, which is considered a red flag signaling emerging disability. Our prior work showed that people with PD experienced a 12% decline in amount of walking over one year ? despite relative stability of motor impairments during that year. Treatment targeting walking, the most rapidly changing aspect of disability in PD, may have the greatest influence on slowing the impact of disease progression on physical function and reducing disability. Traditionally, rehabilitation has targeted impairments and functional limitations with the expectation that gains would translate into greater participation in real-world activities. However, the evidence suggests that this does not occur. In this proposal, we suggest a paradigm shift in which the primary target of the intervention is real- world walking behavior, as greater walking activity could preserve walking function and slow disability. The primary factors that limit engagement in walking in PD are psychological (e.g., low self-efficacy) rather than physical (e.g., motor impairments) in nature. As such we will evaluate a cognitive-behavioral approach, grounded in social-cognitive theory and targeted at enhancing walking activity. Our ?connected behavioral approach? links physical therapists to persons with PD using a mobile health (mHealth) platform to deliver strategies to increase self-efficacy and provide goal-oriented, dynamic walking routines and walking enhancing exercises over one year. We will compare this approach to a control intervention which provides equivalent components and dosing of walking and a walking enhancing exercise program delivered by physical therapists but without a cognitive-behavioral mHealth approach. We hypothesize that the mHealth group will demonstrate higher amounts of walking activity and greater walking capacity relative to the control group. With regard to mechanism underlying improvements in the mHealth group, we hypothesize that self-efficacy will mediate changes in amount of walking and that changes in amount of walking will mediate changes in walking capacity over one year. The insights to be gained regarding mechanisms underlying changes noted will be critical to inform rehabilitation interventions designed to encourage sustained, long-term physical activity. If effective, our ?connected behavioral approach? offers a unique, generalizable and scalable means to increase walking activity and improve walking capacity, thereby reducing disability in PD and perhaps in other chronic progressive conditions.